Process for the production of fibrous refractory oxides of polyvalent elements

ABSTRACT

A METHOD FOR THE PRODUCTION OF OXIDE WHISKERS OF THE ELEMENTS TI, BE, MG, CA, ZN, B, AL, RARE EARTH METALS, SI, SN, V, ZR, SB, CR, MN AND FE OR MIXTURES THEREOF BY CONTACTING A COMPOUND OF THE AFOREMENTIONED ELEMENTS WITH A MELT OF AT LEAST ONE SALT OF A SULFUR-CONTAINING OXYACID CAPABLE OF FORMING A VOLATILE ANHYDRIDE, UNDER THE REACTION CONDITIONS. OPTIONALLY THE MELT CONTAINS ADDITIONALLY AT LEAST ONE METAL HILIDE PREFERABLY AN ALKALI METAL OR ALKALINE EARTH METAL HALIDE. NORMALLY THE REACTION IS CARRIED OUT AT A TEMPERATURE OF BETWEEN ABOUT 600 AND 1200*C. THE OXIDE WHISKERS WHICH FORM IN THE MELT ARE EASILY SEPARATED FROM THE REACTION MIXTURE EITHER BY CENTRIFUGING OR FILTERING THE HOT MELT OR BY REMOVING THE WHISKERS FROM THE REACTION BYPRODUCTS STILL IN THE MELT BY DISSOLVING SAID BYPRODUCTS IN WATER.

UnitedfStates Patent Office 3,684,443 Patented Aug. 15, 1972 3,684,443PROCESS FOR THE PRODUCTION OF FlBRO-US REFRACTORY OXIDES OF POLYVALENTELEMENTS Hans Zirngibl, Duisburg, and Gerhard Winter, Henning Erfurth,and Manfred Mansmann, Krefeld, Germany, assignors to FarbenfabrikenBayer Aktiengesellschaft, Leverkusen, Germany No Drawing.Continuation-impart of application Ser. No. 888,963, Dec. 29, 1969. Thisapplication May 8, 1970, Ser. No. 35,924 Claims priority, applicationGermany, Jan. 2, 1969, P 19 064.7; Aug. 28, 1969, P 19 43 636.3 Int. Cl.C01g 23/04, 1/02, 25/02 US. Cl. 423-612 12 Claims ABSTRACT OF THEDISCLOSURE A method for the production of oxide whiskers of the elementsTi, Be, Mg, Ca, Zn, B, Al, rare earth metals, Si, Sn, V, Zr, Sb, Cr, Mnand Fe or mixtures thereof by contacting a compound of theaforementioned elements with a melt of at least one salt of asulfur-containing oxyacid capable of forming a volatile anhydride underthe reaction conditions. Optionally the melt contains additionally atleast one metal halide preferably an alkali metal or alkaline earthmetal halide. Normally the reaction is carried out at a temperature ofbetween about 600 and 1200 C. The oxide whiskers which form in the meltare easily separated from the reaction mixture either by centrifuging orfiltering the hot melt or by removing the whiskers from the reactionbyproducts still in the melt by dissolving said byproducts in water.

The present application is a continuation-in-part of our prior andcopending application Ser. No. 888,963, filed Dec. 29, 1969 nowabandoned;

This invention relates to a process for the production of fibrousrefractory oxides of polyvalent elements in the so-called whisker form,with an aspect ratio of at least 10: 1, and preferably from 100:1 to2000: 1, or more. In the present context, the aspect ratio is defined asthe ratio of length to diameter of a crystal. The whiskers arefilament-like monocrystals. The whiskers produced in accordance with theinvention have a length of at least about 2000/a and diameters of fromabout less than 1,4 to around and preferably from 1 to 3 By virtue bothof their resistance to high temperatures and their outstanding abilityto reflect infra-red rays, the oxide whiskers that can be produced inaccordance with the invention can be used as insulating materials athigh temperatures. In addition, they are eminently suitable for use asreinforcing components in composite materials by virtue of their extremeaspect ratio, their outstanding strength and their high Youngs modulusof elasticity. The oxide fibres can readily be converted into felt-likemats and by virtue of their outstanding chemical resistance may be used,for example, as a filter material for hot and corrosive liquids andgases.

A process for the production of fibrous oxides of the elements titanium,beryllium, magnesium, calcium, zinc, boron, aluminum rare earth metalssuch as those with atomic numbers of 58 through 72, silicon, tin,vanadium, zirconium, antimony, chromium, manganese, iron and mixturesthereof by reacting a compound of the aforementioned elements Which isconvertible under the reaction conditions to the corresponding oxide,preferably a volatile compound, with a salt of an oxyacid in the melthas now been found in which a metal compound or a mixture of metalcompounds, possibly in admixture with one or more carrier gases, iscontacted at a temperature above 600 C. with a melt of one or more saltsof a sulfur-containing oxyacid which is able to form a volatileanhydride under the reaction conditions, and the fibrous oxide obtainedis subsequently separated from the reaction medium.

It is of advantage to add metal halides, preferably metal chlorides, tothe melt of one or more salts of said oxyacid salts are able to formvolatile anhydrides under the reaction conditions.

Surprisingly, it is readily possible under the conditions according tothe invention to obtain fibrous oxides with an aspect ratio of at least10:1, generally from :1 to 2000:1, without any of the disadvantages ofconventional processes. Oxide needles are obtained which are shown to bemonocrystals (whiskers) by X-ray dilfractions and investigations under apolarisation microscope.

The following table shows the results of two tests, one of which wascarried out with chlorides and the other in the absence of chloridesunder otherwise the same conditions. Tests show that the addition ofchlorides considerably increases the number of needles with a relativelyhigh degree of fineness.

Composition of the melt:

The temperature during the process depends on the composition of thesalt melt and on the chemical reaction which occurs during theconversion of the metal compound to the oxide.

The process is carried out at a temperature of from 600 to 1200 0,preferably from 750 to 950 C. At these temperatures, the melts do nothave a corroding effect upon the container material used, quite unlikemelts based on boron oxide, 'borates, fluorides and complex fluorides.Accordingly, in most cases the process may also be carried out in anapparatus made of ceramic materials.

Another advantage is that, on completion of the reaction, the residualmelt can readily be separated from the fibrous oxide formed. Forexample, the melt can readily be dissolved in cold water. It is alsopossible, however, to separate the oxide fibres formed from thelow-viscosity melt at elevated temperatures.

The oxide whiskers are easy to work up. The residual melt is dissolvedwith cold water or warm water and the whiskers obtained may then bereadily separated from the aqueous phase by filtration or decantation.If desired. the

whiskers may then be divided up into the required fractions either byelution or by screening.

The metal compounds which are to be converted to the oxide may becontacted with the salt melt in solid, liquid or gaseous state, or in acombination of these physical states. The starting materials are notlimited to a certain type of compounds. For the production of fibrousTiO S110 SiO or A1 the corresponding chlorides, fluorides or sulfatesare the most suitable. But also other compounds such as sulfides,hydroxides, double sulfates, nitrides, nitrites, nitrates, carbonates,carboxylates, alkoxides, thiosulfates, 'borates or mixtures thereof maybe used.

The salt melt has to supply the oxygen needed for the conversion of themetal compound to the oxide.

Salts such as sulfates, hydrogen sulfates, polysulfates, thiosulfates,sulfites, disulfites, or mixtures thereof, may be used as theoxygen-yielding components of the melt in the practical application ofthe process. Such salt melt components (as, for example, sulfates) arenormally able to exist-under normal pressureover a broad temperaturerange Without decomposition. If a metal sulfate is used as startingcompound for the production of fibrous oxides it must be decomposable tothe oxide under the reaction conditions.

The following sulfates or combinations thereof have proven to beparticularly effective: Li 'SO Na SO K 80 (11130 MgSO ZnSO CdSO- KAl(SOMnSO; and NiSO The halides of monovalent, divalent and polyvalent metalsmay be used as metal halide components of the melt, provided they do nothave an excessive vapour pressure at the reaction temperature. It hasbeen found that, of the halides, the chlorides are the most suitable.The longest and finest oxide needles grow in melts containing at leastone chloride selected from the group: LiCl, NaCl, KCl, MgCl and CaClOptimum melts for the production of oxide fibres are obtained under thereaction conditions from the combination of the aforementioned preferredsulfates and chlorides. The ratio by weight of the metal sulfate tometal halide may be varied within wide limits. However, a content offrom 65% to 90% by weight of metal halide has proven to be of particularadvantage.

If the starting components are used in gaseous state, e.g. as SnCl SiClTiCl or AlCl -gas, it was noticed that the crystallisation process takesplace primarily on the surface of the melt. In order to ensure that thereaction is as complete as possible, therefore, a large melt surface isoifered to the gas to be reacted. The oxide whiskers are readily formedat a constant melt temperature. Accordingly, there is no need for aspecial temperature program for nucleus and needle growth. The high rateof formation of the oxide whiskers in the melts used in accordance withthe invention is extremely favourable. Experience has shown that areaction time of 1 hour is sufiicient to grow long whiskers in highyields. The yield per unit time is considerably higher than in theprocesses mentioned in the literature.

The process is best carried out at normal pressure, although it may alsobe carried out at elevated pressure or reduced pressure. The quantity ofthe starting materials used can be varied within wide limits. Excessgaseous starting materials can be recovered by condensation of gaseousefiiuent and recycled to the process.

The gaseous starting materials, e.g. TiCl SnCl SiCl or AlCl mayadvantageously be used in admixture with a carrier gas. Nitrogen or air,for example, may be used as the carrier gas, although it is alsopossible to use undiluted gaseous reactants.

The technical terms as used herein are defined by H. W. Ranch et al. inCeramic Fibers and Fibrous Composite Materials, 1968, Ac. Press, NewYork, London.

The process according to the invention is illustrated by the followingexamples:

4 EXAMPLE 1 5.0 g. of a mixture of 60% by weight of Na SO and 40% byweight of Mg'SO were fused in a stream of pure dry nitrogen in anelectrically heated tubular furnace at a temperature of 900 C. Afterthis temperature had been reached and before entering the reaction tube,the stream of nitrogen (40 litres per hour) was passed through a supplyvessel containing TiCl, at room temperature. The TiCh-loaded stream ofnitrogen was allowed to flow over the melt for period of 60 minutes, andthe melt was then cooled in a nitrogen atmosphere. During the reactiontime of 1 hour, the weight of the titanium tetrachloride in the supplyvessel underwent a reduction of 5.2 g. Numerous beard-like, crystallineefllorescences had formed. The cooled reaction product was freed withwater from any melt still adhering to it. Washing with distilled waterand drying at C. left 1.2 g. of TiO whisker in the form of a looseasbestos-like felt which was examined under a microscope and measured.The yield of fibrous titanium dioxide came to around 95% by weight. Therest consisted of isometric particles. The TiO needles had lengths of upto 430 and an average thickness of less than in.

EXAMPLE 2 20 g. of a mixture of 60% by weight of Na S0 and 40% by Weightof MgSO were fused and heated to a temperature of 810 C. in a drynitrogen atmosphere (28 litres per hour) by means of an electriccrucible furnace. The crucible sealed with a close-fitting ceramic platewas provided with a thermocouple, an inlet pipe for nitrogen and for amixture of TiCl and nitrogen and an exhaust pipe. When the melt hadreached a temperature of 810 C., the nitrogen was passed through liquidTiCl at 90 C. before entering the reaction vessel. After 2 hours, thesupply of TiCL, was shut oif and the vessel was flushed with nitrogen.During this period, the weight of the titanium tetrachloride in thesupply vessel underwent a reduction of 46 g.

Working up in the usual way with distilled water left 9.6 g. of TiO;; inthe form of a fairly loose, woolly white product from which thefollowing needle fractions were isolated by elution in water followed byscreening (mesh width and 40 Fraction 1:

Needle length up to 1000p Needle diameter 1 to 2 1. Fraction 2:

Needle length up to 800 i Needle diameter Lu Fraction 3:

Needle length up to 800,1 Needle diameter 1 to la EXAMPLE 3 6.7 g. ofTiCl vapour were passed for 4 hours at 900 C. over 13.0 g. of a mixtureof 30% by weight of Na 'SO and 70% by weight of MgSO The carrier gas wasnitrogen (14 litres per hour). Working up leaves 1.4 g. of TiO Whiskerswhich contain hardly any isometric components. The needles have a lengthof up to 640 and diameters of from 1 to 2 1..

EXAMPLE 4 A mixture of 4.8 g. of Na SO 3.2 g. of MgS-O and 20 g. of'NaCl accommodated in an A1 0 crucible was fused and heated to atemperature of 950 C. in a dry nitrogen atmosphere (40 litres per hour)by means of an electric crucible furnace. The crucible sealed with atightfitting ceramic plate was provided with an inlet pipe for nitrogenand for a mixture of TiCL; and nitrogen and also with an exhaust pipe.The temperature was measured with a thermocouple in the outer wall ofthe crucible. When the melt had reached a temperature of 950 C., thenitrogen was passed through liquid TiCL; at room temperature beforeentering the reaction vessel. After 1 hour, the supply of TiCl was shutoff followed by flushing with nitrogen. During this period, the weightof the titanium tetrachloride in the supply vessel was reduced by 5.2 g.

28 g. of Na SO accommodated in a platinum crucible, was fused and heatedto a temperature of 1200 C. for three hours. The mixture was allowed tocool in the furnace. The cooled reaction product was freed with 5 waterfrom any melt adhering to it. The residue consisted Numerous crystallineefilorescenes had formed on the of TiO -needles with rutile structure.The needles had base and primarily on the inner wall of the crucible.The lengths of up to 700p and an average thickness of becooled reactionproduct was freed with water from any tween 2 and 5,44. The yield offibrous Ti0 came to melt adhering to it. Washing with distilled waterand around 97% by weight; the rest consisted of isometric drying at 120C. left 1.21 g. of TiO' whisker in the particles. form of a looseasbestos-like felt which was examined EXAMPLE 17 under a microscope andmeasured. The TiO needles had A mixture of L3 5 of K2TiF6 and 295 f solengths of P to 2410i and an average thlckness of accommodated in aplatinum crucible, was heated to a EXAMPLE 5 temperature of 1050 C.during two hours. After a reac- 5 9 g of Ticl vapour were allowed toflow for 1 tion time of three hours at 1050 C. the meltwas allowed honiat .900 C. :ver a salt mixture of 4 8 g of Nazso, to cool. The reactionproduct was dissolved in Water and 3.2 g g v 2 g of Licl. and 47 g. ofNaCL isitlrtfigeirghe residue consisted of TiO -needles with rutile :Ihecarrier gas was mtrogen (40 htres per hour). Work- What is claimed is:

35 g if; g ggfpg g l i gig fizg A method for the production of fibrousrefractory len ths of u to 2310 and an average diameter of 2 oxldeWhlslfeis of polyvalent eleme-nts sele-c-ted frqm tile g P groupconslsting of titamum, alummum, silicon, tm, zir- EXAMPLE 6 conium,antimony, iron or mixture thereof which comprises contacting one or morecompounds of said ele- A mixture Of 10 g. of K 5 4, 10 gf Caclz and 10gments with a molten bath of at least one salt of a sulfurof LiCl wastreated with TiCL; vapour under the same containing oxyacid, selectedfrom the group consisting of Conditions as in p -3 g- 0f k P Werebisulfates, sulfates and polysulfates for a time sufiicient passed overthe melt. 0.94 g. of fibrous titanium dioxide to convert said polyvalentmetal to an oxide whisker therewith needle lengths of up to 1590 and anaverage needle of at 600 to 1200" C., and separating ofli the whiskerswidth of 2,14 were obtained from the reaction product. formed in themelt by such contact.

Tem- Dura- Stream ars' 0.2.1: of r s; st Composition of the melt (g.)0.) (hours) i /h gt) W1 1) Example 7 4ZuSO4, 10 C8012, 10 K01 900 1011.1,375 2-3 8 8.57 ZnSO-i, 7.93 KOl 1 40 ca. 2,100 2-3 9 10 Nisoi, 20NaCl 1 40 ea. 1,635 2-3 10.. 4 011s0.,20 NaC 1 40 ca, 1, 687 2-3 114CdSO4, 10 CaC 10 1 40 ca. 1,956 1 12 5.45 KA1(SO4)2, 6.6 09.01;, 13.4NaOl 1 40 ca. 1, 515 1-2 1 Through TiCli at room temperature.

EXAMPLE 13 2. A method as claimed in claim 1 wherein said f n whiskersare titanium dioxide.

in 1 g. of MgSO and 20 g. of NaCl. The carrier gas was addmonauycontauis at .least one nietal hahde' nitrogen (40 litres per hour).Working up leaves 0.25 g. A method as m clam 3 W F nietal hand? 18 offine snO -needles which contain hardly any isometric selepted from} thegrolip conslsting of hthlilm chloride components The needles had lengthsof up to 700 sodium chlorlde, potasslum chlor1de,magnes1um chlorlde, andan average thickness of about 2 1. and mlxtures thereoti In the Samemanner ZrCl4 vapour was passed at 5. A method as 1n claim 1 wherem thereaction is C. over a fused eutectic mixture of Na SO and MgSO earnedout at tempeiaturei of betweenjso and 950 The carrier gas was nitrogen.Working up leaves colour- .method as m clalm 1 i Salt of an lesszrofneedles which had lengths of up to 700. and oxyacid 1s selected fromthe group consisting of sulfates, a thickness of 201980,. hydrogensulfates, polysulfates, thiosulfates and mixtures 1f ZrCl was replacedby a vaporous mixture of FeCl thereof and SbCl red-brown, transparencyneedles were obtained A method as damn I wherem ,alkalme earth at Theseneedles consisting of Fezoa and sbzos metal sulfates or alkali metalsulfates or mixtures thereof had lengths of up to 520/4 and an averagethickness of compnse the moltqn h about 2p A method as in claim 1 wherema m xture of mag- EXAMPLE 14 Irjieslilum sulfate and sodium sulfatecomprise the molten AlCls Vapour were Passed two hours at 9. A method asin claim 3 wherein at least one of the 1 000 C 1 3 tu ur f Over a e 0f10 gof z zsulfates selected from the group consisting of Li SO Thecarrier gas was mtrogen (20 litres per hour). A1 0 N SO K 80 CuSO MgSOZnSO CdSO MnSO needles i needle lengths of #0111 20 t0 and an NiSO andmixtures thereof is used as the oxyacid salt average thlekness 0f wereObtamedcomponent of the molten bath and wherein at least one EXAMPLE 15of the chlorides selected from the group consisting of 35 g. of SiClvapour were passed for one hour at 900 5 g g C3012 and mlxtures thereofB C. over a fused mixture of 6 g. of Na SO and 4 g. of use ast 6 meta ecpmponent' MgSO The carrier gas was nitrogen (10 litres per hour).method as wherem mamum tetra- Working up in the above described way withwater and hahde used as the f i reactant, diluted HCI leaves woollysiorfibrfi '11. A method as m claim 2 wherem titamum tetra- I I chlorideis used as the titanium reactant. EXAMPLE 16 12. A method as claimed inclaim 3, wherein said A mixture of 1.1 g. of TiOSO 1.5 g. of NaHSO andsalt is a sulfate and said halide is a chloride which is ReferencesCited UNITED STATES PATENTS 4/ 196-1 Berry 23-202 3/ 1966 Pease 23-305 83,030,183 4/1962 Berry 2320Z 3,338,677 8/ 19-67 Berry 23202 3,331,6607/1967 Berry 2351 5 EDWARD STERN, Primary Examiner US. Cl. XR,

